Murine leukemia-inducing retroviruses (MuLVs) can undergo changes that allow them to cause neurological diseases in rodents, and understanding the molecular basis for these changes will give us insights into the genetic elements required for distinct disease phenotypes. We have been studying PVC-211 MuLV, a variant of the erythroleukemia-inducing Friend MuLV that causes a rapid neurodegenerative disease when injected into newborn rats. We previously showed that PVC-211 MuLV has an altered host range, allowing it to replicate in rat capillary endothelial cells (CEC) as well as Chinese hamster ovary (CHO) cells, both of which are resistant to replication of Friend MuLV. Using chimeric constructs between PVC-211 and Friend MuLV, we were able to demonstrate a direct correlation between the altered host range of the virus and its ability to cause neurological disease, and the genetic changes responsible for the altered host range were mapped to two amino acids in the SU envelope protein of the virus. To determine if the envelope protein of PVC-211 MuLV was better able to bind to the ecotropic viral receptor (CAT-1) on rat CEC and CHO cells, we constructed mutants of the mouse CAT-1 gene by changing its third extracellular domain (TED), which is crucial for its viral receptor function, to resemble that of the rat CEC or CHO CAT- 1 genes. To facilitate receptor analysis, we fused the wild-type and mutant CAT-1 genes with the jellyfish green fluorescent protein (GFP) gene and transfected the constructs into human 293 cells, which are resistant to ecotropic MuLVs. Our results indicate that 293 cells expressing wild-type as well as mutant CAT-1 genes could be infected with both PVC-211 MuLV and Friend MuLV, indicating that failure of Friend MuLV to infect rat CEC or CHO cells was not due to the inability of the envelope protein of this virus to interact with the TED of the CAT-1 proteins on these cells, at least when they are expressed in human cells. The observation that PVC-211 MuLV has an unusual tropism for CEC not only in the brain, but also in other organs, suggests that the virus may be a useful tool for gene transduction into CEC. We are currently testing whether PVC-211 MuLV can localize to the proliferating endothelium routinely observed in developing tumors. If this is successful, we will use PVC-211 MuLV to target specific genes to tumor endothelium where their expression should selectively kill these cells and result in tumor regression.